Spread spectrum techniques permit data signals to be extracted from noise with a relatively small error rate. In a spread spectrum system, a binary 1 data bit is represented by a first sequence of binary states or elements called chips, and a binary 0 data bit is represented by a second sequence of binary chips. Each sequence of binary chips is known as a spreading sequence. Typically, the second sequence of chips is the complement of the first sequence chips. Thus, in a spread spectrum coding technique, the basic unit of data, i.e. the data bit, is encoded by forming a sequence of chips. The chip rate is much higher than the data rate and each chip has a much broader frequency spectrum than a data bit. Since each chip has a frequency spectrum that is spread much wider than the frequency spectrum of the original data bit, the technique is known as spread spectrum.
At a receiver, a stream of data bits, which has been transmitted using the spread spectrum format, is recovered using a process known as despreading. Despreading involves correlating the received signal which comprises chips, with a local reference chip sequence. The local reference utilizes the same chip sequence that was used to encode the data bits at the transmitting end. More particularly, at the receiver, the reference chip sequence is stored in a register. The signal comprising the received chips is entered into a shift register, and for each chip time period, the received chips are advanced one position in the shift register. At each chip time period, the number of matches between the prestored reference sequence and the received chips in the shift register is obtained. Consider the case where the reference sequence corresponds to a binary 1 data bit and the binary 0 data bit is represented by the complement of the reference sequence. In this case, when the number of matches exceeds a predetermined upper threshold, a data decision is made indicating the presence of a binary 1 data bit. When the number of matches falls below a predetermined lower threshold, a data decision is made indicating the presence of a binary 0 data bit. In between the data decisions times, when there are portions of two consecutive spreading sequences in the shift register (i.e. the end portion of one sequence and the beginning of the next sequence), the number of matches falls in between the high and low thresholds, thus preventing erroneous data decisions.
In a noise free system, the presence of a binary 1 will be indicated by a total match between the received chips in the shift register and the reference sequence. Similarly, the presence of a binary 0 will be indicated by no matches between the received chips in the shift register and the reference sequence. However, in real systems, noise prevents all the chips in a spreading sequence from being correctly received so that binary 1 and binary 0 data decisions are based on whether the number of matches is above or below upper and lower predetermined thresholds, respectively. This process has the effect of averaging out random noise. Thus, the despread signal component is enhanced and the noise component is reduced. This is known in the spread spectrum art as process gain and is defined as the number of chips per data bit.
In short, in a spread spectrum communication system, at an encoder or spreader, each data bit in a data bit stream is coded by transmitting a sequence of binary chips. Typically, one such spreading sequence is used to represent a binary "1" and its complement is used to represent a binary "0". The chips representing the data bit stream are modulated onto a carrier using a conventional two-level modulation technique such as binary phase shift keying (BPSK) or frequency shift keying (FSK), and transmitted to a decoder or despreader. At the decoder or despreader, the chips are demodulated and correlated with a reference sequence to reconstruct the original data bit stream.
In a spread spectrum system, the symbol rate is the rate at which sequences of chips are transmitted and the data rate is the rate at which data is transmitted. For a given symbol rate, it is desirable for the data rate to be as high as possible.
Outside of the spread spectrum art, it is well known that encoding multiple bits per symbol increases the data transmission rate for a given symbol transmission rate, provided that the symbols can be received with a sufficiently low error rate. In the prior art, the encoding of multiple bits per symbol is typically accomplished by various multiple level modulation schemes such as quadriphase shift keying (QPSK) or quadrature amplitude modulation (e.g. 64QAM). These multilevel modulation techniques utilize combinations of changes in phase and/or amplitude to provide multiple (i.e. more than two) modulation levels. Thus, each transmitted modulated symbol can represent more than one bit. The disadvantage of these multilevel modulation schemes is that there is an increase in symbol error rate for a given signal-to-noise ratio as the modulation scheme contains more levels.
In view of the foregoing it is an object of the present invention to improve the spread spectrum communication technique by increasing the data rate for a given symbol rate.
It is a further object of the present invention to improve the spread spectrum communication technique by increasing the data rate for a given symbol rate by encoding multiple bits with each spreading sequence.
It is also an object of the present invention to improve the spread spectrum communication technique by increasing the data rate for a given symbol rate without the use of a multilevel modulation technique.